The invention relates to a plasma picture screen with a front plate comprising a glass plate on which a dielectric layer and a protective layer are provided, a back plate, a number of gas-filled plasma cells arranged between said plates and separated by partitioning walls, and a plurality of electrodes on the front plate and the back plate for generating corona discharges.
Plasma picture screens can generate color pictures with high resolution and have a compact construction. A plasma picture screen comprises a hermetically closed glass cell which is filled with a gas, with electrodes in a grid arrangement. When a voltage is applied, a gas discharge is generated which emits light in the ultraviolet range. This UV light is converted into visible light by phosphors and emitted through the front plate of the glass cell to the viewer.
Two types of plasma picture screens can be distinguished in principle: a matrix arrangement and a coplanar arrangement. In the matrix arrangement, the gas discharge is ignited and maintained at the intersection of two electrodes on the front and back plate. In the coplanar arrangement, the gas discharge is maintained between the electrodes on the front plate and is ignited at the intersection with an electrode, a so-called address electrode, on the back plate. The address electrode in this case lies below the phosphor layer. This arrangement implies that half the UV light generated in the gas discharge will arrive at the front plate, where it is absorbed in the layers present there. This effect is even enhanced for a portion of the UV light, because the UV light is re-absorbed in the gas space in that gas atoms are excited from the ground state into a higher energy level. The light is indeed emitted again subsequently, but it is diverted from its original direction, so that also light which had originally been directed towards the phosphor layers can arrive on the front plate.
The luminance of a plasma picture screen depends to a major extent on the efficacy with which the UV light excites the phosphors. To increase the luminance, T. Murata, Y. Okita, S. Kobayashi, T. Shinkai, and K. Terai describe a plasma picture screen in IEEJ, 1998, EP 98-61, wherein the front plate comprises not only a dielectric layer and the discharge electrodes, but also a UV light reflecting layer. A similar construction of a front plate was described by the same authors in IDW, 1998, 539-542. In this case the front plate in addition comprises a protective layer of MgO, and the UV light reflecting layer is present between the dielectric layer and the protective layer. In either case said layer has the task of reflecting UV light emitted in the direction of the front plate towards the phosphors.
The invention has for its object to provide a plasma picture screen with an improved luminance.
This object is achieved by means of a plasma picture screen with a front plate comprising a glass plate on which a dielectric layer and a protective layer are provided, a back plate, a number of gas-filled plasma cells arranged between said plates and separated by partitioning walls, and a plurality of electrodes on the front plate and the back plate for generating corona discharges, characterized in that a UV light reflecting layer is provided on the protective layer.
Contrary to the front plate described in IDW, 1998, 539-542, the UV light reflecting layer does not lie between the dielectric layer and the protective layer. This has the advantage that the UV light does not pass through the protective layer, but is reflected directly at the lower surface of the front plate. An absorption of the UV light in the protective layer is prevented thereby. The additional protective layer in the plasma picture screen according to the invention as compared with that of IEEJ, 1998, EP 98-61 protects the subjacent layers from the high ignition voltages and temperatures which are required for a plasma discharge and arise in the plasma discharge, respectively.
It is particularly preferred that the UV light reflecting layer comprises oxides of the composition M2O, such as Li2O, or oxides of the composition MO, with M chosen from the group Mg, Ca, Sr, and Pa, or oxides of the composition M2O3, with M chosen from the group B, Al, Sc, Y, and La, or oxides of the composition MO2, with M chosen from the group Si, Ge, Sn, Ti, Zr, and Hf, or oxides of the composition Mxe2x80x2Mxe2x80x32O4, with Mxe2x80x2 chosen from the group Mg, Ca, Sr, and Ba, and Mxe2x80x3 chosen from the group Al, Sc, Y, and La, or fluorides of the composition MF, with M chosen from the group Li, Na, K, Rb, Cs, and Ag, or fluorides of the composition MF2, with M chosen from the group Mg, Ca, Sr, Ba, Sn, Cu, Zn, and Pb, or fluorides of the composition MF3, with M chosen from the group La, Pr, Sm, Eu, Gd, Yb, and Lu, or fluorides of the composition Mxe2x80x2Mxe2x80x3F3, with Mxe2x80x2 chosen from the group Li, Na, K, Rb, and Cs, and Mxe2x80x3 chosen from the group Mg, Ca, Sr, and Ba, or phosphates of the composition M3PO4, with M chosen from the group Li, Na, K, Rb, and Cs, or phosphates of the composition M3(PO4)2, with M chosen from the group Mg, Ca, Sr, and Ba, or phosphates of the composition MPO4, with M chosen from the group Al, Sc, Y, La, Pr, Sm, Eu, Gd, Yb, and Lu, or phosphates of the composition M3(PO4)4, with M chosen from the group Ti, Zr, and Hf, or metaphosphates with a chain length n of between 3 and 9 and the composition (MxPO3)n, with x=1 if M is chosen from the group Li, Na, K, Rb, and Cs, x=xc2xd if M is chosen from the group Mg, Ca, Sr, Ba, Sn, Cu, Zn, and Pb, x=⅓ if M is chosen from the group Al, Sc, Y, La, Pr, Sm, Eu, Gd, Yb, and Lu, and x=xc2xc if M is chosen from the group Ti, Hf, and Zr, or polyphosphates with a chain length n between 101 and 106 and the composition (MxPO3)n, with x=1 if M is chosen from the group Li, Na, K, Rb, and Cs, x=xc2xd if M is chosen from the group Mg, Ca, Sr, Ba, Sn, Cu, Zn, and Pb, x=⅓ if M is chosen from the group Al, Sc, Y, La, Pr, Sm, Eu, Gd, Yb, and Lu, and x=xc2xc if M is chosen from the group Ti, Hf, and Zr, or primary phosphates of the composition MH2PO4, with M chosen from the group Li, Na, K, Rb, and Cs, or NH4H2PO4, or diamond.
Particles with these compositions show no or only a small absorption in the wavelength range from 147 to 700 nm and withstand the high temperatures prevailing during manufacture of a plasma picture screen. In addition, these particles can be manufactured with particle diameters between 10 nm and 500 nm.
It may be preferred that the UV light reflecting layer comprises particles with a particle diameter between 200 nm and 500 nm.
Particles of these diameters show a substantially greater light scattering in the UV wavelength range than in the visible wavelength range.
It is preferred in this embodiment that the UV light reflecting layer has a thickness of 0.5 xcexcm to 5 xcexcm.
Besides the scattering characteristics of the individual (isolated) particles and their wavelength dependence, the thickness of the layer of scattering particles also plays a part. Thus the reflection of a layer of particles which are less strongly scattering can be high if the layer thickness is great. The use of particles with a particle diameter between 200 nm and 500 nm and a layer thickness of 0.5 xcexcm to 5 xcexcm results in a UV light reflecting layer which reflects strongly in the wavelength range of the plasma emission and transmits the visible light emitted by the phosphors.
It may also be preferred that the UV light reflecting layer comprises agglomerates of particles having particle diameters between 10 nm and 200 nm.
Particles with particle diameters between 10 nm and 200 nm do not scatter UV light, so that layers made from such particles show no appreciable reflection. If it is ensured by means of suitable measures that the particles form agglomerates which are substantially greater than 100 nm, the layer will behave optically as if it were a layer of larger particles. The interaction between the light and the agglomerates is determined both by the size of the agglomerates and by the particle diameters of the particles forming the agglomerates. The variations in density and refractive index in the agglomerates also has an influence on the wavelength dependence of the reflection.
It is preferred in this embodiment that the UV light reflecting layer has a thickness of 0.2 xcexcm to 10 xcexcm.
It is furthermore preferred that the UV light reflecting layer covers the protective layer completely or only partly.
A partial covering of the protective layer with the UV light reflecting layer already leads to a considerable improvement in the luminance. It may be advantageous for the plasma discharge if the protective layer, which is usually made of MgO, is covered only partly. This is because the voltage required for igniting the plasma is reduced by the MgO layer.